The present invention is directed to x-ray sources, particularly to miniature x-ray sources, and more particularly to millimeter scale size x-ray sources using hot filament cathodes and which are capable of producing broad spectrum x-ray emission over a wide range of x-ray energies.
X-rays are typically produced in an assembly consisting of a vacuum housing, a high voltage feedthru, a high voltage connection to the feedthru, an anode, and a cathode. The system operates by applying high voltage across a gap between the anode and the cathode. Electrons are emitted by the cathode and accelerated by the high voltage to the anode. The electrons impact on the anode and create both broadband and line radiation through collisional excitation of the atoms within the anode. The x-rays are emitted from the anode and propagate through the vacuum housing to be used for any given application. The vacuum housing must be highly transparent to the x-rays or have a highly transparent window mounted in the vacuum housing to allow the x-rays to escape for use. The entire assembly is under vacuum to allow for efficient transport of the electrons from the cathode to the anode and prevent a gas discharge or an electrical arc from forming, which would significantly reduce the voltage across the gap (reducing the x-ray energy) or destroying the assembly. Efficient production of x-rays and the production with sufficient energy (5-40 keV or higher) for a wide range of applications requires the use of high voltages (5-40 kV or higher). The fabrication of compact x-ray sources has not been readily attainable until recently due to the need to have high voltage over extremely small dimensions (.about.1 mm), high vacuum in extremely small volumes (cubic mms), and a high voltage connection of extremely small size (.about.1 mm). Recently a miniature x-ray source which overcame the above-referenced limitation has been developed, and such is described and claimed in copending U.S. application Ser. No. 09/391,578, filed Sep. 8, 1999, entitled "Miniature X-Ray Source." In that miniature x-ray source, field emission type or cold cathodes were utilized.
The present invention constitutes an improvement over the miniature x-ray source of the above-referenced copending application by the use of hot filament cathodes. By way of example, a prototype hot filament cathode was tested with a 1-2 volt, 0.5-1.0 amps current used to heat a 25 micron diameter tungsten filament. The hot filament emits electrons which are then accelerated to the anode by the high voltage applied across the gap. Tests have shown that the miniature (millimeter scale size) x-ray source is capable of efficient production of x-rays with sufficient energy (.about.5-40 keV) for a wide range of applications which require the use of high voltages (.about.5-40 kV).